Apparatus for coating particulate material by thermal evaporation



Aug. 12, 1958 c. A. BAER ETAL 2,846,971

APPARATUS FOR comma PARTICULATE MATERIAL BY THERMAL EVAPORATION Filed Feb. 2a, 1956 FIG. 2

INVENTORS Ckwle; A. Back By Rebel-I W. Shel q,

ATTORNEY United States Patent APPARATUS FOR COATING PARTICULATE MATERIAL BY THERMAL EVAPORATION Charles A. Baer, Needham, and Robert W. Steeves, Nahant, Mass., assignors to National Research Corporation, Cambridge, Mass, a corporation of Massachusetts Application February 23, 1956, Serial No. 567,357

4 Claims. (Cl. 118-49) This invention relates to coating and more particularly to vacuum deposition coatings wherein a coating is deposited in a vacuum and condensed on a substrate consisting of a finely divided material.

It is a principal object of the present invention to provide a new and improved method for uniformly coating all the surfaces of a finely divided material.

Another object of the present invention is to provide a new and improved means for the deposition of a coating of a desired thickness on a substrate consisting of a finely divided fibrous, spherical or powdered material.

Still another object of the invention is to provide a greatly improved means for preparing abrasive particles for bonding.

Still another object is to provide means for coating material uniformly with conducting or nonconducting coatings.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the product possessing the features, properties and the relation of components and the process involving the several steps and the relation and the order of one or more of such steps with respect to each of the others which are exemplified in the following detailed disclosure, and the scope of the application of which will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawing wherein:

Fig. 1 is a diagrammatic view of an embodiment of the invention; and

Fig. 2 is an enlarged diagrammatic view of Fig. 1 taken along the line 2-2.

Vapor deposition coating of numerous materials has become commercially Well known. However, the difficulties in uniformly coating finely divided particles have never been eliminated, although a number of patents indicates that the problem has received considerable attention. In the present invention, these difliculties of the prior art are overcome by providing a vacuum chamber in which the finely divided substrate is directed past a source of vapors of a coating material While being held in a circumferential path by centrifugal force. This is preferably achieved by providing a rapidly rotating drum against the inner surface of which the finely divided substrate is held by centrifugal force. This finely divided substrate may be fibrous, granular, powdered or the like and can be composed of any material which can be coated by vacuum deposition techniques. For example, suitable substrates are materials such as acetate flock, granular silicon carbide, small ceramic objects and powdered metals.

In the utilization of the apparatus described above, the vacuum chamber, after insertion of the substrate and the source or sources, is evacuated to below 100 microns, the exact pressure depending on the specific material being evaporated, its vapor pressure and the temperature.

2,846,971 Patented Aug. 12, 1958 The coating material, which may be composed of a pure metal such as aluminum, copper, zinc, chromium and lead or a compound such as silicon oxide, is heated to a temperature high enough to cause its evaporation. This temperature varies according to the vapor pressure of the specific material in question and the pressure of the system. The drum is then rotated by means of a variable speed motor at a speed sufficient to cause the individual particles to adhere to the inner surface of the drum. By regulating the revolutions per minute of the drum and taking into account the rate of evaporation and the mean free path of the source material at specific temperatures and pressures, the depth of the coating on the substrate material can be controlled. Uniform coatings have been obtained with controlled thicknesses ranging from a few millionths of an inch to a few ten thousandths of an inch.

Referring now to Figs. 1 and 2, wherein like numbers refer to like elements, there is shown a preferred embodiment of the invention. In these figures, 10 represents a vacuum-tight housing defining therewithin a vacuum chamber 12 arranged to be evacuated by a vacuum pumping system schematically indicated at 14. Within this chamber, the substrate 16 to be coated is placed within the drum 18. A means 17 for evaporating the coating ported in the drum 18 by means of the leads to the coil 17b. The crucible holds a charge of the coating material and is heated to a temperature high enough to cause vaporization thereof. During the process, the drum is rotated by a variable speed motor, schematically indicated at 19, at a speed suiiicient to cause the finely divided substrate 16 to adhere to the inner wall 20 of the drum due to centrifugal force. A scraper bar 21 is installed adjacent the inner wall 20 in such a way as to cause the substrate to tumble, thereby exposing all surfaces. This bar is positioned at the bottom of the drum and slightly off center in the direction opposite to the direction of rotation of the drum so as to cause tumbling Without causing violent swirling, which would tend to precipitate some of the substrate into the coating material and thus cause contamination of the source. A deflector blade 22, supported by the scraper bar 21, is positioned in the opening of the drum 18 in order to deflect particles 16 back into the drum and to thereby prevent particles 16 from falling out of the drum.

In order to describe the invention more fully, a nonlimiting, illustrative example of the invention is set forth below.

Example I A charge of finely divided acetate fibers was placed in an iron drum having an internal diameter of 15% inches, a length of 14 inches, and a wall thickness of A inch. This drum was placed in a horizontal position upon two rollers situated in parallel and 9% inches apart, each roller having a diameter of 1% inches and one of said rollers being driven by a variable speed motor.

A source material consisting of aluminum was placed in a molded carbon crucible, having an internal diameter of 2% inches and a depth of 1% inches, and was subsequently suspended Within the drum. The entire apparatus was placed within an air-tight system, which system was evacuated to 0.18 micron and the temperature raised to about 1250 C. by an input of 6 /2 kilowatts. The drum was then rotated at 138 revolutions per minute for the duration of the run. The electrical resistance of the coated fibers was subsequently measured and found to be in the range of 2.3-3.8 ohms per square.

It should additionally be pointed out that a suitable source'could consist of a plurality of individual sources or a single line source or merely means for introducing coating vapors. In a preferred embodiment of this invention, a continuously fed source is employed such as that illustrated in U. S. Patent 2,643,201.

Since certain-changes maybe made in the above product and :process withoutdeparting from the scope of the invention therein involved, it is intended that all matter containedin-the above description or shown in the accompanying drawing shall be interpreted as illustrative and not "in a limiting sense.

What is claimed is:

1. Apparatus of the type in which a coating is applied to discrete particlesofa material, the improvement Which comprises [means forholding said particles in a circumferential course by centrifugal forceiabove asource material, means for reducing the pressure within the system, means for heating said source material to a temperature high enough to cause vaporization of said material at the existing pressure, and means for causing said discrete particles to tumble, thereby exposing all surfaces to the coating vapors.

2. Apparatus for coating discrete particles of a substrate material which comprises a drum enclosed within a vacuum system, means for evacuating said system, means .-for rotating said drum about its horizontal axis at a speed sufiicient to hold said particles against the surface of the drum While they are carried in a circumferential path over a means for generating vapors of a coating material, and means for tumbling said particles.

3. The apparatus of claim 2 wherein the means for generating .vaporsof a coating material comprises a crucible containing .said coating material arranged to be heated by an induction coil within the drum,

4. In apparatus for coating discrete particles on all sides thereof by a vapor deposition process of the type wherein the coating material is evaporated under a high vacuum and vapors of the coating material are condensed on the surface of the discrete particles, the improvement which comprises a generally cylindrical supporting chamber positioned inside of an evacuated chamber, the supporting chamber being mounted for "rotation around its axis, such axis being generally horizontal, a source 'for holding material to be evaporated inside of said cylindrical chamher, said source being arranged toe-vaporate the "coating material in -'a generally upwardly direction, means for rotating said cylindrical chamber at a speed sufiiciently high to 'c'ausethe discrete particles to be held by centrifugal force in contact with "the inner surface of the supporting chamber and to be carried over the coating source so as to be contacted by the coating vapors, and means for tumbling the-discrete particles at anotherposition'inltheir travel to provide afor substantially uniform coatingon all surfaces of the particles.

References Cited in thefile of this patent UNITED STATES PATENTS 1,695,362 Bourges Dec. 18, 1 928 2,161,950 Christensen June 13, 1939 2,378,476 Guillich June '19, 1945 2,398,517 Castor Aprfil'6, 1946 2,601,355 Wuyss e't 'zil June 24, 1952 2,639,269 Dube May '19, 1953 2,658,847 'MCDon'ld 'Nov. 10, 1953 2,702,523 Prestwood 22,1955 

